Structural studies aimed at providing insight into the mechanism of methane oxidation by methane monooxygenase were continued. A second crystal form of the hydroxylase component of soluble methane monooxygenase from Methylococcus capsulatus (Bath) was solved to 2.4 E resolution. Methane access to the active site was also investigated by pressurizing the crystals with xenon gas. Xenon, which is a good mimic for methane owing to the similarity in size, was found to bind in a hydrophobic pocket located approximately 12 E from the active site.